Low frequency projector or hydrophone



2 Sheets-Sheet l W. P. MASON Low FREQUENCY PROJECTOR 0R HYDROPHONE Filed Dec. 2s, 1942 F/Gl @kf/,amy

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o/ELEcrn/c coNsrM/r 00000000 numuwma4a mmf/:Nar /N k/LacrcLEs /N VEA/TOR WPMASO/V Patented Aug'. 6, 1946 I LOW FREQUENCY PROJECTOR OR HYDROPHON E Warren P. Mason, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 28, 1942, Serial No. 470,398

Claims.

This invention relates to multiunit radiating andV receiving devices and to high power compressional wave radiating devices. In the hereinafter described preferred embodiments, illustrative of the principles thereof, it relates particularly to radiating and receiving devices employing a large number of piezoelectric crystals which are capable of radiating high power compressional energy waves and to compressional wave energy radiators and receivers which have prismatic properties.

By way of definition, in the present specification a prismatic device, for other than light energy waves, should be understood to be a device which in transmitting a wave comprising energy of numerous frequencies within a particular frequency spectrum will spread the frequency spectrum by imparting a direction, differing for each frequency, to the several frequencies of the spectrum or which in receiving energy will respond to the several frequencies of the spectrum only when they approach the device at particular respective angles, differing for each frequency.

The prismatic characteristics of the devices of the present invention are similar to those of the devices of my copending application entitled '-Pipe antennas and prisms, filed March 1, 1941, Serial No. 381,236.

The object of the invention is to provide an efficient low frequency submarine projector or hydrophone which may lbe used for transmitting and receiving signals in a range where conditions are more favorable than for the ordinary devices used for this purpose.

In accordance with the present invention a submarine projector or hydrophone is formed of a mosaic structure of piezoelectric crystals in the form of a thin blanket of considerable area attached to the hull of a ship. Such a blanket consists of a plurality of narrow vertically disposed units connected in an electrical network by which a prismatic effect may be achieved and including amplifying means adjusted to suppress secondary lobes. By way of example, such a blanket may be of the order of three feet in height, seven and a half feet in length and an inch and a half in thickness.

Each of the said units consists of an array of bimorph crystal structures of a size and shape best suited for the frequency range desired. Such a plurality of crystals are electrically connected in parallel and physically housed in a single container whereby in form each unit effectively consists of a long, narrow and thin strip.

A feature of the invention is a hydrophone consisting of a blanket-like structure comprising a plurality of crystals divided into groups, each group being connected into an electrical network providing means for achieving a prismatic eiect and the suppression of secondary lobes, the separate crystals .being constructed and arranged to respond to frequencies within the audible range.

The drawings consist of two sheets having fourteen figures, as follows:

Fig. 1 is a fragmentary view of a plurality of radiators showing how they may be mounted on the hull of a ship;

Fig. 2 is a cross-sectional view taken on line 2, 2 of Fig. 1 of a crystal showing how the crystal may be mounted and illustrating the relationship of the oil and air chambers with respect thereto;

Fig. 3 is a similar View showing another form of crystal arranged to be insensitive to longitudinal waves in the metal frame;

Fig. 4 is a fragmentary circuit diagram showing an electrical circuit equivalent to the bimorph crystal;

Fig. 5 is a similar but more detailed circuit;

Fig. 6 is a circuit diagram showing the relationship of the equivalent constants for a crystal of a particular size as derived in the body of the specification;

Fig. 7 is a circuit diagram showing a half section of a confluent type filter illustrating the manner of using the electromechanical driving elements;

Fig. 8 is a fragmentary side view of the hull v of a ship showing the placement of the radiator in relation to the water line;

Fig. 9 is a schematic circuit diagram showing how a plurality of radiators are connected in a network with a plurality of filters to produce a prismatic effect;

Fig. 10 is a schematic circuit diagram showing how unidirectional amplifiers may be used in the network of Fig. 9;

Fig. 11 is a circuit diagram showing a circuit arrangement by which the amplifiers may be pointed in one direction or the other as the crystal devices are to be used as radiators or hydrophones;

Fig. 12 is a graph showing the result of calculations of the radiation impedance of an iniinite cylinder;

Fig. 13 is a pair of graphs showing fundamental constants of a i-degree X-cut Rochelle salt crystal vibrating in flexure; and

Fig. 14 is a pair of graphs showing the eiiiciency of conversion of a bimorph Rochelle salt projector used at low frequencies. The solid line graph shows th'e projector designed as a lter circuit and the dotted line graph shows an alternative wide band low efficiency design.

This disclosure sets forth a method for obtaining an ecient low frequency projector or hydrophone which uses l5-degree X-cut Rochelle salt crystals in the form of a bimorph unit. Calculations indicate that such a unit will cover a frequency range from 2750 cycles to 6000n'cyc1eswith better thanv a 50 pery cent efficiency ofconversion. By resonating the static capacity at a lower frequency than the reso-nant frequency of the crysv-4 tal, it is possible to cover the frequency range from '700 cycles to 7000 cycles with a 10 per cent`' eciency of conversion.

A low frequency radiator of this sort/can be` used as an element in a prism type projector and receiver for submarine location. Due :to thev low frequencies possible, the medium will have less attenuation and less refractivitys and hence it is possible to locate a submarine at greater distances and with a greater certainty. Such a projector also h'as the advantage that on account of the lower frequency a pulse sent out from'the searching ship will not be as easily located by a submarine. A K

In the low frequency range from v1000 to 6000 cycles, it is dicult to obtain a projector or hydrophone which will radiate .or` pick up. sound with a good efficiency. Magnetic projectors made in this range have an efficiency of lessthan 1 per cent and, furthermore, have a small power Vh'andling capacity. The ,device of the present invention is a projector or pick-up devicel using bimorph Rochelle salt crystals capable of radi-y ating a wide band of frequencies efficiently and with vconsiderable amounts of power. g i Y In form, the radiator consists of a longY strip, by way of example, '7 to 8 centimeters widegfto 4 centimeters thick and several feet long. Inthe drawings, Fig. l illustrates how a plurality of such radiators may be attached to the external surface of the hull of a ship. The same showing is made in Fig. 8Y to illustrate'the 'position Vof the plurality of radiators with respect -to therwater `line. In Fig. l, each radiator comprises a frame I housing a plurality of bim'orph crystals' 2; indicated by a plurality of horizontal linesf YThe various units may be secured tothe surface of the hull of the ship in any convenient manner, such', for instance, as by a plurality of.Y clamps 3 which may be loosened and rotated'to allow the removal and replacement'of anyone unit, at a time.

Such a unit can be used as an efficient pick-up device having a directivity normal to the length thereof but no directivity along other directions. By mounting a number of `such radiators'side by side, attached directly to the ships hull and conT nected to a suitable filter or phase shifter a prism-type projector and hydroph'onevcan be obtained capable of workingrin the low frequency range. Due tothe lower attenuation and re' fraction of the medium for such frequencies the range of such a device may be larger than for the ultrasonic type of radiator. Furthermore, the position of a ship sending out a low frequency pulse is not as easily located by a'fsubmarine unless it also has a large receiver capable of working to low frequencies. Y

On accountof the birnorphtype f construction used, such a device mounted in'fthe side of va ship will not pick up the structural noise propagated through the frame of the ship.`

Onerform of the low frequency projector is shown in Fig. 2. Y It consists of a steel framework comprising the elements 4 and 5 which, when assembled in any convenient manner, securely h'old the bimorph crystal. This crystal consists of two slabs of crystals 'I'and 8, plated ontheir surfacesgwith conducting material. The conducting surfaces of the outer faces of the crystal are connected together and form one electrical tersurfaces Aare connected together andjconnected to conductor I2 which forms the other terminal of the device. As shown, the outer surfaces are electrically connected to the frame but it will be understood Ythat the device may easily be completely insulated from the frame.

In the complete unit consisting of a plurality of crystals the side.

crystal chamber.

il electrical-connections of such terminals are in ,multipla 'sow that-each radiator will have two terminals. v

-On th'e radiating or upper side, the chamber between the crystal and the rubber Vdiaphragm I3 is filled with castor oill I4, which `conducts the vibrations from the crystal to the rubber diaphragm, which in turn conducts the vibrations to the seawater.` vOn the other side ofv thecrystal is an air chamber I5 whichintroduces a low Vimpedance on the crystal. [Since the tWo sides of the chamber arein seriesin the equivalent electrical circuit, all of the energy will be transferred to the seawater sideandnone to theair When the radiator is 'lowered intoV the water, the air chamber is compressed but maintains an equalizing pressureon the crystal surface and prevents the crystalfrom being bent.

In order to prevent the leakage of airthrough the rubber diaphragmV I I, lathin metal diaphragm 9 is soldered to the metal frame 5 on both sides .of the chamber. In order to prevent the leakage of castor oil to the crystal .a similar thin metal diaphragm I!) islsoldered to the frame part 4. In

lchamber isvthen filled vwith castor oil and' the upper rubber diaphragm 'is screwed'in place. It will vbe noted that aV thin strip of rubber I6 between the upper and lower metal frame parts 4 and 5 lprevents sea water Vfrom coming into the This is crushed dcwv'nY until a metal-to-metal contact is made between the two parts 4 and 5 of the frame. l

When it is desired to have the pick-up insensitive to longitudinal waves in the metal frame,

as would be the; casey if the projector is mounted Vin the ships frame, the construction shown in Fig. `3 may be used. For this case the crystal is 'surrounded on both sides by oil chambers which have small interconnecting holes which allow theloil to flow at a slow rate from one chamber to therother. In this construction there is a bottom frame member Il' and an upper frame member I8. A thin metal ydiaphragm I9 is soldered to the lower frame member I'I and the crystal of twoslabs 2l) and 2| is clamped between the frame members. The chambers between the rubber diaphragmVZ-E and the crystal 2| and the crystal 20 and the metal diaphragm I9 are lled with castor oil. An air chamber is provided between the metal diaphragm I9 and the metal backing plate 2'3. Small by-pass holes 24 and 25 are provided to allow communicationbetween the two chambers 'filled with castor oil. At very rapid frequencies these small openings act'like choke coils' and do notV allow a rapid interchange of oil from one chamber t'o the other. AtV a high water pressure, the rubber diaphragm is pressed in and more oil flows to the back oil chamber causing the air chamber to compress. 'In lthis way no static pressure acts on the crystal. -When a longitudinal vibration actuates the frame of the projector approximately'equal pressures are generated in the two Oil chambers on each side of the bimorph crystal. As a result, since the crystal is responsive only to unbalanced pressures, no voltage will be generated by such a vibration.

Calculation of radiation eiciencies The equivalent circuit of a bimorph crystal of this type has been worked out for a unit which is supported on the two ends but is free to bend. This corresponds closely to the case considered here since the glue will prevent the ends from moving but is not stiff enough to keep them from bending. The equivalent circuit as shown in Fig. 4 relates the input voltage and current into the crystal to the average pressure and volume Velocity over the surface of the crystal. In this equivalence Co is the static capacity of the crystal, CivrVV the acoustic compliance of tne'crystal electrically short-circuited, i. e.. the ratio between the average pressure over the surface to the time rate of displacement of the crystal surface, and M is the effective inertance of the crystal. In centimetergram-second electrostatic units these quantities have the values,

where l, Zw and Zi are respectively the lengths, width and total thickness of the bimorph crystal, K is the longitudinally clamped dielectric constant of the crystal, Y0 the value of Youngs modulus of the short-circuited plated crystal, p the density of the crystal and D a function of the piezoelectric moduli.

The complete equivalent circuit for the crystal used on a radiator is shown in Fig. 5. The impedances on the two sides of the crystal are effectively in series. On the non-radiating side the impedance will consists of the inertance of the oil chamber in series with the compliance of the air chamber. This compliance is so large compared to the compliance of the crystal that it can be neglected. On the radiating side the elements consist of the inertance of the oil charnber and the radiation resistance and inertance of the medium. For a circular source the resistance for small apertures increases proportional to the square of the frequency, but for a long thin source it is to be expected that the resistance would vary more nearly proportional to the frequency. This has been ooniirmed by calculations on the radiation resistance of infinite and nite cylinders vibrating radially. Such calculations show that if the length of the cylinder is greater than half a wave-length at all frequencies concerned, the resistance and reactance of a radiating cylinder vary as shown in Fig. l2. The dotted line lshows the multiplying factor plotted as a function of wR/c, to correct the radiation resistance for the effect of fmite length, where w=21rf; a=radius of cylinder; lzlength of cylinder; A: wave-length; p--density of water; and C==velocity of sound in water.

We are interested primarily in the radiation resistance and reactance of a pulsating rectangle in an infinite balile. However, comparison between radiating spheres and radiating pistons in baiiles shows that if the ratio of the diameter to the wave-length is less than two tenths, the

radiation resistance and reactance for the sphere and piston do not differ appreciably. Hence, We are justified in assuming that the radiation resistance and reactance of a rectangle a half Wavelength long or greater over the frequencies of interest will have the same resistance and reactance as a cylinder having the same diameter as the rectangle is wide. This results in a radiation resistance of the right order of magnitude to work well with the impedance of a bimorph Rochelle salt crystal.

The most advantageous type of crystal for this use is a L5-degree X-cut Rochelle salt crystal. Although it cannot radiate as much power as a ll-degree Y-cut crystal, it has a higher electromechanical coupling Which results in a considerably :wider frequency range than can beradiated when Y0 is the value of Youngs modulus for an unplated crystal. Yo', the value for the plated and short-circuited crystal, is given by when the clamped dielectric constant, is as shown in Fig. 13. With this value of K the eiective Youngs modulus for a plated crystal is as shown in Fig. 13 as a function of temperature. Since most of the projectors are worked below 18 C., we can take as rough values Yo'=2.2 1011; K=150 By way of example, let it be assumed that it is wished to radiate a band of frequencies centered around 4000 cycles. It is assumed that 6 centimeters is about the longest l5-degree X-cut crystal that is commercially feasible. The resonant frequency of the crystal in air will be f: 1 ='.455l, [Xi

In water, however, we add masses due to the water load and the Weight of the oil in the chambers. If We make these chambers a total thickness of .5 centimeter on a side or Ztl-1.0 centimeter, the added inertance due to the oil will be 1.10 i l w olb- Zlw The added inertance of the water as determined from Fig. 12 will be u., 21r 400o u.,

Actually an allowance of B/Zlw is made since the piston type usually has more added mass than the cylinder type. Hence, the total inertance added to the inertance of the crystal should be 7 The resonant frequency will then be R 21a/m l2 p 1 +3.?

Inserting the values Yo'=2.2 1011; p=1.775

this becomes 1.6 105it 1 fR- z2 1+1.83/l, (lo) If the band is centered at 4000 cycles, the resonant frequency of the crystal should be 4000 cycles. Assuming 1:6. centimeters and solving for Zt, we nd R=.58 pc=8.7 104 acoustic ohms per square cm. (13) In acoustic impedance units we have to divide this by the cross-sectional area which is 6 Square centimeters, so

R=1.45 104V ohms, acoustic resistance. (14) If we take these quantities through the electromechanical transformer, we will have electrical quantities as shown on Fig. 6, having the values for a crystal 1 centimeter Wide :238 mmf.

is then a half section of a coniiuent type which has the element values v- Taking the ratio of Cn to C1, and the product .L1C1,we have Solving the rst equation, using the value 3.17 for Cri/C1, We find f2=1ye`8f1 18) The second equation shows that the crystal resonance occurs at the mean frequency of the pass band which in this case is 4000 cycles. Hence, for this case ,fr-:3040 cycles fz=5270 cycles The image impedance at the main frequency should be Zo=21r(f2-f1) L1=2.96X105 ohms per unit length (20) which agrees Well with the as calculated hereinbefore The actual loss in conversion from electrical to mechanical energy can be calculated by solving for the current in the output of the network of Fig. 7 and comparing that to what would radiation resistance L =M 2:21.?, h 15 l /q 4 elmes be obtained by the use of a perfect transformer 133:@ :2 84X105 Ohms between the input resistance RA and the output radiation resistance RB. This results v1n the The filter-type structure is the most eiiicient equation Y Q NRA-RB Y (2i) i RA 2 i-aLlo1 RARB(1-@Lco)]2 \/[R..+RB M001@ @213.00m @2130001 w01 1 *do way to utilize electromechanical driving elements, since the lter with matched impedance terminations is the device which will deliver to its output all the energy sent into it over the widest If we insert the lter relations hereinbefore stated in relation to Fig. 7, and let RA, the out-v put impedance of the amplifier equal Z0 for the lter, we have Y where mM =ZT1 m =the mean frequency of the radiator A plot of this equation with Re taken from Fig. l2 and ence-,226

'9 is then shown in Fig. 14. As can be seen, the efficiency of conversion does not drop below 6 decibles or 25 per cent from 2400 cycles tov 6000 cycles.

By separating the resonance of the electrical coil with crystal capacity from the mechanical resonance, a somewhat wider band can be obtained as shown by the dotted line graph of Fig. 14 at some sacrifice of efciency of conversion. It can be shown that the best eiciency will be obtained by letting the right-hand terms of Equation 21 vanish when w2=wAwB is the resonance of the coil and condenser, and we the me chanical resonance of the crystal and added mass. This results in the relation hull of a ship, and .the change in direction is obtained by prism methods, the device becomes practical.

As an example let us consider what can be done -with a radiator working from 2.75 kilocycles to 6 kilocycles, as calculated previously. To get the same directivity as at ultrasonic frequencies will require a radiator 6 wave-lengths long which at the mean frequency of 4000 cycles will be a length of 225 centimeters or 7.4 feet. If we divide this up into long radiators which are .4 of a wave-length apart at the top frequency of 6000 cycles, this requires twenty-two strips each 1.0 centimeters wide. The length of the strips is determined by the directivity in a vertical plane. A length of three feet would concentrate most of the power in an angle 115 degrees from the horizontal. Such a blanket radiator 7.4 feet long, three feet wide and approximately 1.5 inches thick could be attached to the hull on each side of the ship as shown in Fig. 8. On account of the smooth surface, streamlining is obtained and a freedom from turbulent noise. Leads from each radiator connect to a filter or phase shifting net- 2 2 T taie-(eared aan[acera-air As an example, let us take fA=1000 cycles, lo

RB: 1.72 105 electrical ohms (25) Using the same constants as in the examplel considered above, the best amplier impedance is RA=2.07 106 electrical ohms per centimeter width of crystal 26) With these values, and taking Re from the curve of Fig. l2 the efficiencies of conversion in decibels below perfect efiiciency of conversion are shown by the dotted line of Fig. 14. From 700 cycles to 6500 cycles an efficiency of conversion averaging around 10 per cent is obtained with a variation of about i4 decibels over this region. On account of the large area and relatively high efficiency of conversion such a device would provide a more efiicient listening device than has previously been obtained for his frequency range. To work -down to 700 cycles a device about 105 centimeters or 3.5 feet long would be required. This would have a receiving area of 630 square centimeters and would be able to deliver 5 10-'1 watts electrical power to a receiver when a pressure of 100 dynes per square centimeter was acting on it. Since a telephone receiver has a minimum power of about 4X l014 watts to reach audibility, this would represent enough power to operate the receiver without the need of an amplier. F01 such a hydrophone, the static capacity of the crystals would be 25000 ccf, so the additional capacity put on by a cable would be small, and could be neglected. The electrical impedance of an amplifier or receiver to work With the unit should be about 20000 ohms.

Use of low frequency projector in a. prism type detection system In order to make full use of the lower attenuation and refractivity of the lower frequency range, a radiator sufficiently large to obtain a good directivity may be used. Such a radiator is too large to turn manually. If, however, it is used in the form of a thin skin attached to the At the mean frequency of 2000 work as sh'own in Fig. 9 for a listening system. On account of the low power carrying capacity of coils and condensers used in ilters or phase shifting networks it is desirable to place a power amplier between each radiator and the filter as shown in Fig. 10. By adjusting the gain of each amplifier it is possible to compensate for the loss in filter or phase shifting network and moreover secondary lobes can be suppressed by driving the middle segments harder than the' outer segments. Since ampliers are one-way devices it is necessary to have the input connected to the phase shifter and the output connected to the radiator when a pulse is sent and reversed when a listening condition is desired. If the input and output impedances of the amplier are made the same, this can be accomplished by a simple switching arrangement as shown in Fig. 11.

Even if the power radiated by each unit is kept down to the 0.1 watt per square centimeter radiated from other i5-degree X-cut Rochelle salt crystals, the power radiated from each strip will be watts. For all twenty-two strips the total power would be 3.3 kilowatts which is considerably more acoustic energy that has been radiated from ultrasonic projectors. This coupled with the lower attenuation and lower refractivity realizable at the lower frequencies, and the freedom from noise due to streamlining indicates that submarines could be located at farther disstances and under more exacting conditions for the low frequency radiator than for the ultra- Isonic radiator. Such a system also has the advantage that on account of the lower frequency the direction of a pulse sent out from the searching ship will not be as easily located by a submarine.

What is claimed is:

1. A sonic device for use as a submarine projector or hydrophone consisting of a blanketlike array of vertical and horizontal rows of coordinately arranged piezoelectric crystals for placement on the external surface of the hull of a ship, 'said crystals of each vertical row constituting a unit, an electrical network for connecting said array of crystals to transmitting and re- -ordinately arranged piezoelectric 11 ceiving apparatus, said network including phase shifting circuits, each said unit being electrically separated from other of said units by said phase shifting circuits, and each said crystal comprising a bimorph structure constructed and arranged to respond to frequencies within the audible range.

2. A sonic device for use as a submarine pro- 'jector or hydrophone consisting of a blanket-like 'transmitting and receiving ap-paratus, said network including phaseshifting circuits, each said unit being electrically separated from other of said units by said phase shifting circuits whereby a prismatic eiect is secured, an amplifier foreach said'unit, the gains of the amplifiers being higher for the middle units and lower for the end lunits whereby secondary lobes are suppressed, Yand each said crystal comprising a bimorph structure constructed and arranged to respond to frequencies within the audible range.

3. A sonic device for use as a submarine projector or hydrophone consisting of a blanketlike array of vertical and horizontal rows of cocrystals for placementv on the external surface ofthe hull of a ship, said crystals of each Vertical row being connected in multiple, housed in a single container and constituting a unit, an electrical network for connecting said array of crystals to transmitting and receiving apparatus, said network including phase shifting circuits, each said 1 unit being electrically separated from other of said units by' said phase shifting circuits where"- by a prismatic effect is secured, an vampliierlfor each said unit, the gains of the ampliiiers being higher for the middle units and lower for' the end units whereby secondary lobes are suppressed, means for directively pointing said .arnplii'lers in accordance with the directionof use of said device, and each said crystal comprising a bimorph structure constructed and arranged to respond to frequencies within the audible range.

4. A sonic device for use as a submarine projector or hydrophone consisting of a blanketlike array of piezoelectric crystals in vertical and horizontal rows, the crystals of 'each vertical row being connected in multiple and acting as a unit, an electrical line including a series of phase shifting networks with a connection to each said Vertical row of crystals made to a point in said line between succeeding networks, whereby diflerent frequency currents connected to said line will cause correspondingly diierent frequency signals to be transmitted in correspondingly different directions or whereby incoming signals may be received from correspondingly different directions in accordance with theirn frequency. Y

5. A submarine projector or hydrophone consisting of a plurality of bimorph crystals responsive to frequencies within the audible range,

means coordinately arranging said crystals in a sheet of Vertical and horizontal rows, means whereby the crystals in each Vertical row are connected in parallel and comprise a unit, means whereby the various units are connected to a transmission line and phase shifting net-works interposed in said line electrically between each of said units to produce a prismatic effect.V

" WARREN P. MASON. 

